Set LINK_FLAGS for INTERFACE libraries in cmake

Question

I am working on a header-only C++11 library which uses modern CMake. By "modern," I mean not only using CMake v3.0+ but also trying to use as much as possible the best practices in Daniel Pfeifer's talk.

I have done some research on my question, but the answers are mostly regarding modifying the LINK_FLAGS directly in the global level, which I do not want to have. Right now, in my project, I require a minimum version of 3.9.0 of CMake because of some features I am using.

My question is about whether/how to add LINK_FLAGS coming from two of my dependencies: BLAS and LAPACK. Basically, I have the following excerpt from my CMakeLists.txt file:

cmake_minimum_required(VERSION 3.9.0)

project(polo VERSION 1.0.0 LANGUAGES C CXX)

find_package(BLAS REQUIRED)
find_package(LAPACK REQUIRED)

add_library(polo INTERFACE)
add_library(polo::polo ALIAS polo)

target_compile_features(polo INTERFACE cxx_std_11)

target_include_directories(polo
  INTERFACE
    $<BUILD_INTERFACE:${polo_SOURCE_DIR}/include>
    $<INSTALL_INTERFACE:include>
)

target_link_libraries(polo
  INTERFACE
    ${BLAS_LIBRARIES}
    ${LAPACK_LIBRARIES}
)

set_property(
  TARGET
    polo
  PROPERTY LINK_FLAGS
    ${BLAS_LINKER_FLAGS}
    ${LAPACK_LINKER_FLAGS}
)

As far as I can understand from documentations of the FindBLAS and FindLAPACK modules, I need to inform my users at least about {BLAS,LAPACK}_LIBRARIES and {BLAS,LAPACK}_LINKER_FLAGS. For the former, I think I have handled the issue properly. However, for the latter, I need to use either set_target_properties or set_property. Between the two, the latter seems to give me a cleaner solution in that I can use both variables coming from Find{BLAS,LAPACK} modules together. When I try to build my library using the above solution, I get the obvious error:

CMake Error at src/CMakeLists.txt:32 (set_property):
  INTERFACE_LIBRARY targets may only have whitelisted properties.  The
  property "LINK_FLAGS" is not allowed.

My question is two folds:

1. Should I use *_LINKER_FLAGS coming from the modules at all, and,
2. If yes, how should I integrate them cleanly into my CMake project?

As for the 2. above, I have seen some suggestions/answers for using target_link_libraries, but I am not sure whether that is the option to go for.

Thank you for your time!

Answer 1

First of all, I do apologize to the community for cross posting the issue.

Matthieu has tried helping me with two options:

1. Providing a helper function so that the consumers of the library could call the function to properly handle the LINK_FLAGS, and,
2. The IMPORTED library option, which he has kept as the final answer (please see the comments there for the motivation).

Unfortunately, neither of these solutions seem to work. The first one is not a clean way of informing the consumer about your dependencies. The second version seems to work with INTERFACE libraries, but any consumer that depends on the INTERFACE library that build an object, such as, e.g., a C-API of the header-only library that builds a SHARED library, has problems building and installing the IMPORTED library.

The solution seems to be to use CMake v3.13 and above, which, as of the posting date, is in the release candidate (rc3) state. Apparently, CMake v3.13 will be introducing INTERFACE_LINK_OPTIONS for such purposes.

EDIT. CMake v3.13 has been released.

Answer 2

The nice thing is that you can provide a helper .cmake for them (called polo-config.cmake).

One option inside the .cmake file is to create an IMPORTED library, which you hold the flags that you want, probably as PUBLIC this time, so that they are propagated to the next user.

Of course, you need to add the library properly, setting up the include paths, the path to the library...